Rapid frequency shift traveling-wave tube



March 31, 1959 R. D. wx-:GLIN

RAPID FREQUENCY SHIFT TRAVELING-WAVE TUBE Filed Apr-111, 1957 Wi/vraiRolf D.Weg|ein,

y gli" (96m.

United States Patent .IO

Rolf D. Weglein, Los Angeles, Calif., assignor to Hughes AircraftCompany, Culver City, Calif., a corporation of Delaware ApplicationApril 1,1957, serial No'. 649,835

-s claims. (ci. B15- 3.5)

This invention relates generally to traveling-wave tubes and moreparticularly to a traveling-wave tube which is capable -of shiftingrapidly from one set of operating conditions to another.

It is often desired to shift the operating conditions of atraveling-wave tube as rapidly as possible as, for example, shifting thefrequency of a backward-wave oscillator by applying a step of voltage tothe electron stream accelerator. Heretofore, there has been found toexist a relatively long period of time before the new condition has beenestablished, i.e., the new frequency. Delay times or lag times of theorder of 50 microseconds have previously been presumed to be inherent inconventional backward-wave oscillators. It is considered that this lagtime represents the period required for the electron stream to chargethe dielectric such as glass in the proximity of the slow-wave structureand which may be thought of as partially filling the inter-face space ofthe capacitor represented by the slowwave structure as one plate, withother and distant elements comprising the opposite plate.

The problem has been approached in the past by minimizing this capacityas, for example, by spacing other conducting elements further from theslow-wave structure. The results of such an approach have been toachieve lag times only as low as 50 microseconds.

It is an object of kthe Ipresent invention to provide a traveling-waveytube' which mayshiftoperating conditions with much less lag time thanwas heretofore possible. ,p l

It is a further object to provide a traveling-Wave tube such as abackward-wave oscillator in which frequencyl of oscillation may bevoltage shifted in a period .of the order of 5 microseconds.

vBrielly, these and other objects are here achieved in the followingmanner:`

A new approach is utilized to substantially eliminate the dielectriccharging which gives rise to the lag time. Instead of attempting tominimize the capacitance between the slow-wave structure and the outerelements, a shield is provided which isolates the slow-wave structureand the dielectric to be charged from the inter-capacity space. In oneembodiment, a nonmagnetic conductive tube is disposed closely about theslow-Wave structure, for example, just outside the glass envelope. Thisshield is electrically connected to the slow-wave structure in a mannerto insure that both slow-wave 4structure and shield remain at exactlythe same direccurrent potential. An alternative arrangement utilizingdielectric supporting rods between the slow-wave structure and the glassenvelope comprises a conductive coating on the supporting rods and onthe interior of the glass envelope. In both embodiments a shield isprovided which does not appreciably affect the propagationcharacteristics of the slow-wave structure.

Further objects and advantages of the present invention will becomeapparent to those skilled in the art upon Patented Mar. 31, 1959 2 areading of the more detailed description of the accompanying drawing inwhich:

Fig. lis a partially cut-away schematic view of va backward-waveoscillator constructed in accordance with the present invention; v

Fig. 2 is a sectional view of a portion of the midsection of atraveling-wave tube embodying the present invention; and

Fig. 3 is a sectional view of a portion of a third ern bodiment of theinvention.

Referring to the drawing, and particularly to Fig. `1 in which there isshown an example of the present invention in the form .of abackward-wave oscillator 10 which includes aglass envelope 12 supportingand enclosing within its enlarged left end an electron gun 14, andWithin its elongated righthand portion a slow-wave structure which inthe example chosen is'a helix 16. At the right-hand extremity of glassenvelope 12 and sealed thereto is a collector electrode 18.

Closely encompassed about the elongated, more slender portion ofenvelope 12 and extending somewhat beyond the ends of slow-wavestructure 16 is shown a conducting shield 20 which is directly connectedto slowwave structure 16 by a short lead 22. Shield 20 may be lightweight and thin as, for example, a length of nonmagnetic, metallic foilwrapped about envelope 12, and which may be of copper or aluminum. Anoutput lead 24 is connected to the left extremity of slow-wave structure16 and is adapted to be coupled in a conventional manner to atransmission line or waveguide as illustrated. Electron gun 14 includesa cathode 26 with a heater 23 for emitting a stream of electrons whichare focused by a beam-forming electrode 30. An accelerating electrode 32is maintained at a high positive potential with respect to cathode 26and serves to accelerate the electron stream to a high velocity. Thecathode 26 and electrodes 30, 32 may be shaped to develop a hollowelectron beam asillustrated.

Slow-wave structure 16 is ordinarily maintained somewhat higher inpotential than accelerating electrode 32 and its potential with respectto cathode 26 may determine the frequency of oscillation ofoscillator`10.' An adjustable contact 34 connected to a source ofpotential 36 schematically indicates that oscillator 10 is thus tunableby varying the voltage of slow-wave structure 16 with respect toelements of electron gun 14. Collector electrode 18 is shown connectedalso to voutage source 36 at a point a few hundred volts positive withrespect to slow-wave structure 16 whereby the effects ofsecondarily-emitted electrons may be minimized by holding them nearcollector electrode 18. An outer solenoid 38, concentric with respect tothe longitudinal axis of slow-wave structure 16 and energized by asource of potential 40, provides a steady axial magnetic field forconstraining and confining within slow-wave structure 16 the electronstream as it traverses from electron gun 14 to collector electrode 18.

Fig. 2 illustrates a second embodiment of the present invention in whicha slow-wave structure 42 is supported within a glass envelope 44 by aset of dielectric rods 46. This technique of support is conventional andmay apply, as does this example of Fig. 2, generally either toforward-wave or to backward-wave types of traveling- Wave tubes. Rods 46and the interior surface of glass envelope 44 are coated with a thinresistive coating 48. The criteria for this coating are that aneffective capacitive shielding should be provided about slow-wavestructure 42 while not appreciably affecting the radio frequencypropagation characteristics thereof. The coating should be thin comparedto the depth of skin elect at the microwave frequency being propagatedand would ordinarily be less than of the order of .0001 of an inch.Also, the resistance between turns of slow-wave structure 42 should beseveral orders of magnitude greater than the characteristic impedance ofthe tube such as to present substantially an open circuit to the energybeing propagated. An effective coating for such purposes is stannouschloride evaporated onto rods 46 and the interior surface of envelope44. The coatings on the rod must be in good electrical contact with thecoating on the envelope to provide the prescribed shielding effect.

Referring to Fig. 3, Athere is shown a portion of the mid-section of atypical ribbon helix forward-wave travcling-wave tube amplifier inwhich, as similarly shown in Fig. 1, ribbon helix 50 is supported by theglass envelope 52 while a conductive shield 54 is provided about theexterior surface of envelope 52 which is directly connected to helix 50by a short lead 56.

In operation, when the velocity of the electron stream or operatingparameters are changed for the purpose of changing the frequency ofoperation, no dielectric charging or recharging is required before thetube attains the new steady state condition. In Fig. 1 the dielectric ofenvelope 12 is effectively removed from the inter-capacity space betweenslow-wave structure 16 and outer elements, such as solenoid 38 andwaveguides or any cooling means. This is accomplished by shielding theslow-wave structure by shield 20 and maintaining it through lead 20 atthe same direct-current potential as that of slow- .wave structure 16.

In the example of Fig. 2, dielectric charging or recharging of envelope44 and rods 46 is minimized by conductive, resistive coating 48 whichshields slow-Wave structure 42 from all the dielectric involved. Theshield 54 of the example of Fig. 3 operates in the same manner as doesshield 20 in Fig. l. In both cases, as with Fig. 2, dielectric chargingwhich gives rise to the time lag in arriving at a new steady-statecondition of operation is minimized or substantially eliminated by ashielding of the dielectric from the inter-capacity space between theslow-wave structure and the outer elements of the traveling-wave tubecircuitry.

What is claimed is:

l. A traveling-wave tube providing a minimum lag time in attaining achange in state of operation comprising: a slow-wave structure; anelectron gun for projecting a stream of electrons in energy exchangerelationship with said slow-wave structure; dielectric support means formaintaining said slow-wave structure in alignment with the electronstream; and shielding means having a finite electrical conductivitydisposed about and substantially encompassing said slow-wave structureand electrically directly connected thereto in a manner to shield saiddielectric support means from the inter-capacity space between saidslow-wave structure and outer capacitive elements, whereby the chargingof said dielectric by said electron stream is substantially eliminated.

2. A backward-wave oscillator traveling-wave tube providing a minimumlag time in attaining a change in frequency of oscillation comprising: aslow-wave structure; an electron gun for projecting a hollow stream ofelectrons in energy exchange Arelationship with said slow-Wavestructure; dielectric support means for maintaining said slow-wavestructure in alignment with said electron stream; and shielding meanshaving a nite electrical conductivity disposed about and substantiallyencompassing said slow-wave structure and electrically directlyconnected thereto in a manner to shield said dielectric support meansfrom the inter-capacity space between said slow-wave structure and outercapacitive elements whereby the dielectric charging of said supportmeans by said electron stream is substantially eliminated.

3. The device as set forth in claim 2 in which said dielectric supportmeans comprises a glass envelope contiguously surrounding said slow-wavestructure, and said shielding means comprises a hollow nonmagneticconductive cylinder disposed contiguously about said glass envelope andwhich is electrically directly connected to said slow-wave structure.

4. The device as set forth in claim 2 in which said dielectric supportmeans comprises a glass envelope and a plurality of elongated dielectricrods disposed substantially symmetrically on the interior surface ofsaid glass envelope in a manner to support said slow-wave structurewithin said envelope radially spaced therefrom by a distance equal tothe diameter of said elongated rods; and said shielding means comprisesa thin coating of resistive material substantially over the interiorsurface of said envelope and said elongated rod in a manner to provideelectrical connection between said slow-wave structure and saidshielding means.

5. The device as claimed and set forth in claim 4 in which said coatingcomprises an evaporated coating of stannous chloride which has athickness of the order of .0001 of an inch.

References Cited in the tile of this patent Peter Nov. 12, 1957

